For many years it has been common practice to utilize ultrasonic surface waves to inspect the surfaces of forged and cast steel rolling mill work rolls for the presence of defects such as firecracks and spalling. Such defects occur as a result of normal wear and tear through routine usage in the hot strip mill. Typically, an ultrasonic beam is introduced into the surface of the roll at the critical angle of approximately 58 degrees or greater, which results in the generation of surface waves which generally travel circumferentially along the surface of the roll. The waves are highly influenced by the presence of very small defects, cracks, and pores, as well as dirt, grease, and water on the roll surface. These conditions cause a portion of the surface wave to be reflected back, and received by an ultrasonic wave transducer which generates the wave. This is known as the pulse-echo ultrasonic inspection technique. Such known practices are disclosed in a paper by N. C. Nelson and C. O. Zamuner, entitled "Surface Wave Ultrasonic Inspection For The Detection and Removal of Cracks in Indefinite Chill Double Poured HSN Work Rolls" proceedings of AIME Mechanical Working and Processing Conference, Montreal, October 1992.
In common practice, the method of performing this type of test is strictly manual. First, the work roll must be in a finished or semi-finished ground state such as by roll grinding. The roll is then set down on chalks or neck supports so that access can be made to the entire roll body. An important and necessary step is to wipe and dry the ground roll to remove all surface contaminants before inspect in.
A bead of ordinary oil or ultrasonic couplant is then applied along the top of the roll from end to end on the roll body. The manually held surface wave transducer is placed in contact with the stationary roll on the area of oil and oriented to propagate the wave circumferentially around the roll body. The transducer is then moved across the body of the roll.
The ultrasonic instrument sensitivity and range may be adjusted so that the interference echo at 180 degrees roll circumference is seen at 80 to 100% of full screen on the instrument base line and at an amplitude of 100%. Any defect indications would be seen between the initial pulse and the interference echo. Typically these will appear as echoes on a cathode ray tube (CRT) display. This gives only a general indication of the location and type of defect on the roll surface. In order to locate the indicated defect, a manual step must be performed.
It is customary to locate the actual area where the echoes are generated by moving an oil covered finger in the path of the sound beam until the echo from the finger is coincident with the echo from the surface of the roll. If the echo remains after wiping off the suspect area with the finger and a rag then it is most likely a fire crack in the roll surface. If the echo disappears, then the indicated defect was only debris or contaminate on the roll surface.
After half of the roll has been inspected by this tedious technique, the roll is again cleaned and a new bead of oil applied. The transducer is manually reversed to circumferentially inspect the other 180 degrees of the roll surface and the above roll is rotated 90 degrees after both halves have been inspected, wiped down again and the whole process repeated. This last step is to insure that there are no uninspected areas on the roll, specifically, interference echo at 180 degrees.
After such inspection, if crack remnants have been found, the work roll must be loaded back into the roll grinder, usually resulting in more wasted time in waiting for the crane operator and the time to manhandle the roll back into the grinder. The roll must then be ground again to remove the surface damage, and then reinspected. This results in loss of time and productivity, and the added expense of the grinding operations.
What is needed is a method and apparatus for improving the efficiency of inspecting work rolls by reducing the time for inspections and for minimizing costly time spent on the grinding equipment. The method and apparatus should be done dynamically on a rotating work roll instead of stationary rolls and should minimize false echoes created by contaminants and debris on the roll surface. The invention should be useful in conjunction with the roll grinding equipment in order to minimize roll handling operations.